This invention relates to solid-state lasers, and more particularly to an active mirror amplifier (AMA) laser having a generally thin laser gain medium attached by a pressure differential to a rigid substrate cooled by a flow of cooling medium through microchannels incorporated therein, thus enabling a construction of a laser capable of producing high-average power with good beam quality.
High-power solid-state lasers are finding increasingly important applications in defense and commercial applications. The most recent growth in solid-state laser business can be attributed to the introduction of diode pumping. Advantages of solid-state lasers are all-electric operation, wavelength suitable for transmission through optical fibers, continuous duty capability, high wall-plug efficiency, and the possibility of engineering a high-power device into a small, lightweight package. For these reasons the commercial market for high-power (i.e., greater than 200 watts) solid-state lasers has grown steadily for the last decade. Potential military applications have also become more important in recent years. Most applications of high-power solid-state lasers require good beam quality. Beam quality (xe2x80x9cBQxe2x80x9d) is a measure of how well the laser beam can be focused to a spot. BQ is critically important in laser weapons where concentrated optical energy is required to thermally damage a specific target (e.g., a missile in flight). Other military applications also require good BQ for certain types of laser illuminators and other imaging-related uses. Similarly, laser beams with good BQ are required for industrial applications to produce high quality, deep penetration welds and precision cuts at increased speeds. Furthermore, availability of a low-cost, high-power solid-state laser with good BQ would open the door to extensive new applications, such as welding of aluminum in manufacture of light-weight automotive bodies, and cutting and drilling of aluminum and titanium in the production of aircraft.
Present day solid-state lasers extract coherent light from an inverted population of neodymium, ytterbium, or other suitable ions doped into crystals or glass. Population inversion is achieved by optically exciting dopant ions by absorption of optical radiation at wavelengths shorter than the laser wavelength. This process is commonly referred to as xe2x80x9cpumping.xe2x80x9d Depending on the excitation source and the laser ions used, much of the optical pump radiation is converted into heat and deposited into the solid-state laser medium. Thus, for continuous operation, waste heat must be removed in real time by cooling selected surfaces of the laser medium. Because solid-state laser media are dielectrics that typically have a low thermal conductivity, a significant thermal gradient is created between the hot interior and the cooled outer surfaces. This causes a change in the index of refraction (thermal lensing), thermal expansion and mechanical stress in the medium, medium depolarization, detuning, and other undesirable effects, with possible consequences of BQ degradation, reduced laser power, and possibly even fracture of the solid-state medium. In particular, optical distortions caused by temperature gradients transverse with respect to the laser beam optical axis are known to reduce BQ.
Consequently, efficient heat removal and reduction of thermal effects caused by temperature gradients across the active area of the laser medium usually dominate design considerations for high-average power continuous wave (CW) solid-state lasers. Recently introduced pumping by narrow band radiation from laser diodes matched to absorption lines of dopant ions greatly reduces the amount of waste heat dissipated in the laser medium. Nevertheless, major heat-related problems in existing solid-state lasers are limiting their operation at high-average power and good beam quality.
With prior art solid-state, high-power lasers, several techniques have been introduced to reduce temperature gradients and/or mitigate their effects on laser operation. Chernoch, in U.S. Pat. No. 4,233,567 (1980), discloses a laser medium configured as a slab cooled on large flat sides and with the laser beam traversing the slab in zigzag fashion, as shown in FIGS. 1a-1c. In this concept, thermal gradients in the transverse direction parallel to the large flat sides of the solid-state medium are essentially eliminated and the gradient in the other transverse direction is reduced. Furthermore, the zigzag path of the laser beam through the slab generally averages out local thermal gradients. However, despite the inherent advantages (at least on a conceptual level) of a zigzag slab to mitigate thermal problems and nearly 20 years of engineering development, the acceptance of this type of system has been slow. The reasons for this include low efficiency, residual distortion (especially near the slab ends) which limit BQ, high cost of fabrication, and power scaling limitations.
Another class of solid-state laser amplifiers known as xe2x80x9cactive mirror amplifierxe2x80x9d (AMA) has been investigated in the prior art. Originally disclosed by Chernoch in U.S. Pat. No. 3,525,053 (1970), large-scale laser systems employing AMA technology have been constructed for inertial fusion research. See for example, J. A. Abate et al., Active Mirror: A Large-Aperture Medium-Repetition rate Nd:Glass Amplifier, Applied Optics, volume 20, no. 2, pages 351-361, (1981). In the AMA concept, a single large aspect ratio, free-suspended disk is optically pumped and cooled from the back side, and the laser radiation to be amplified enters from the front, as shown in FIG. 2. The front face of the disk has an anti-reflection coating for the laser radiation, whereas the backside has a dichroic coating, which is highly reflective for the laser radiation and transparent to the pump radiation. Flashlamp pumping is commonly used with the AMA. Advantages of the AMA are:
The pump radiation source can be closely coupled to the laser gain medium;
The laser gain medium is uniformly pumped across the gain profile;
Surfaces receiving the highest heat deposition are cooled by direct contact with flowing liquid;
Double pass configuration compensates for thermally induced birefringence; and
Suitability for circularly polarized beams.
Lasers using AMA were mainly single shot (low-average power) systems (such as used in inertial confinement fusion research) where real-time heat removal was not required. Prior art lasers using AMA are entirely unsuitable for high-average power operation, however, because of several reasons. For one, to ensure structural rigidity, the solid-state disk must be made relatively thick (i.e., several centimeters), which impedes heat extraction. Another reason is that one sided heating of the free-suspended disk causes mechanical distortion resulting in a wavefront error. Yet another reason is that coolant pressure must be low to avoid distortion of the disk, resulting in low flow rates and low heat transfer coefficients. Still further, coolant flow induces vibrations in the disk. Previous attempts to mitigate these problems and increase the average power output of a laser using an AMA have been met with encouraging but limited results.
In recent years, the AMA concept has been revived in the form of a Thin Disk Amplifier (TDA) introduced by Brauch et al in U.S. Pat. No. 5,553,088. The TDA offers significant improvement over prior art lasers using an AMA as it allows operation at significantly higher average power (several hundred watts) and with good BQ. See, for example, H. Hugel and W. L. Bohn, Solid State Thin Disk Laser, SPIE Proceedings, volume 3574, pages 15-28, (1998). The TDA, as shown in FIG. 3, consists of a thin disk (i.e., a crystal) of suitable solid-state laser medium (e.g., Nd:YAG, Yb:YAG) attached to a heat sink by a thermally conductive bond. The rear face of the disk has an optical coating exhibiting very high reflection at the laser wavelength and the pump radiation wavelengths, whereas the front face has a coating with antireflection characteristics at laser and pump radiation wavelengths. Unlike the traditional AMA approach, optical pump radiation is injected into the disk through the front face. Heat generated within the disk is conducted to the rear face and transported into the heat sink through the thermally conductive bond (typically an indium foil).
While the TDA configuration is generally effective in avoiding excessive transverse temperature gradients, it is not scalable to high-average powers (i.e., much greater than several hundred watts). The problem is both thermal and mechanical. With increasing pump power, the disk temperature rises. Although the bond is made of thermally conductive and mechanically compliant material (typically indium), thermal expansion of the disk introduces significant thermal stresses in both the bond and the laser gain medium. Besides distorting the laser gain medium (i.e., thus degrading the laser BQ), such stresses would eventually either damage the bond or fracture the disk. As a result, laser operation at high power density and high BQ is compromised. The problem becomes more severe with increased diameter of the disk. Thus, the total power output from a single disk is limited. In particular, thermo-mechanical considerations have limited the size of the TDA disk to less than 1 cm in diameter and less than 1 mm in thickness.
Furthermore, introducing pump radiation to the front face of the TDA requires optical elements which may interfere with laser beam propagation and laser beam optics. The problem is made more severe if multiple re-injection of the pump radiation is required to compensate for the low absorption of pump power in a disk that is several hundred micrometers thick, as illustrated in FIG. 4. As a result, the laser system is more difficult to package and align. Introducing pump radiation via optical fibers attached to the circumference of the disk is one alternative described by Brauch et al., but this is cumbersome and impedes tight packaging of laser components. In summary, the prior art TDA has the following limitations:
disk diameter (and thus laser power) are limited by thermal stresses to less than about 10 millimeters;
Indium bond is relatively weak and susceptible to failure at high thermal stresses
conductivity variations of the bond joint cause xe2x80x9chot spotsxe2x80x9d in the laser gain medium;
high temperature gradients together with the semi-rigid bond joint cause thermal stresses, which limits pump and laser power, beam quality, and the size of the disk;
limited power output with good beam quality (typically less than 200 watts); and
pump power delivery means are not suitable for tight packaging and easy alignment.
Another approach to handle thermal load problems in a solid state laser gain medium uses the heat capacity (i.e., thermal inertia) of the gain medium to temporarily store dissipated heat. Such a concept, also known as the xe2x80x9cheat capacity laserxe2x80x9d, (HCL) has been disclosed by Albrecht et al. in U.S. Pat. No. 5,526,372. HCL uses one or more free suspended disks or slabs of solid state laser gain medium approximately 1 cm thick with the large faces generally normal (including a Brewster angle) incidence with respect to the amplified laser beam. Prior to laser operation, the laser gain medium is cooled by a flow of gas to initial operating temperature. During laser operation, the laser gain medium gradually warms up until it reaches its final operating temperature. At that point the laser operation is suspended and the laser gain medium is allowed to cool again to its initial operating temperature. After reaching this temperature, the process can be repeated. In this fashion, the HCL can be operated in a semi-continuous fashion. The length of the laser cycle depends on the amount of the laser gain medium and its thermal storage capacity, while the length of the cooling cycle depends on the effectiveness of the gas cooling applied to the large faces of the laser gain medium.
In preferred embodiments the present invention is directed to a laser module having one or more active mirror amplifiers (AMA) which is capable of operating at high-average power and providing near diffraction limited beam quality (BQ). In particular, the preferred embodiments of the present invention provide numerous advantages over prior art AMA designs, including:
AMA with laser diode pump means that reduces the waste heat load to the solid-state laser medium;
a relatively thin solid-state medium to allow efficient conduction of waste heat;
microchannel cooling means for efficient removal of waste heat;
a substrate which provides rigid mechanical support for the solid-state laser medium; optionally, the substrate being transparent to pump radiation;
pressure means to maintain the solid-state gain medium attached and conformed to the substrate;
attachment means that reduce thermally induced stresses in the solid-state gain medium; and
a pre-formed, solid-state laser gain medium that suffers reduced thermally induced stresses during operation.
In one preferred embodiment, the AMA comprises a rigid substrate having a plurality of microchannel passages formed therein and opening to an outer surface. A laser gain medium is disposed against the outer surface and held against it by a pressure differential existing between the atmosphere in which the laser module is disposed and the pressure in the microchannel passages. Such pressure differential depends on the operating conditions but is typically several tens of pounds per square inch (PSI). The dimensions of the microchannels formed at the outer surface of the substrate may vary considerably, but in one preferred form these microchannels have a width of between about 0.005 inch to 0.040 inch and cross-sectional area of between about 0.0000025 to 0.0015 square inch (0.00016125 to 0.01032 square cm).
Furthermore, the rigid substrate contains a heat exchanger suitable for circulating liquid cooling medium. Heat generated in the laser gain medium is conducted into the substrate and transferred into the cooling medium. In this manner, heat that is generated in the laser gain medium is efficiently dissipated in real time during the use of the laser module without significantly affecting the beam quality of the produced laser beam.
In another preferred embodiment, the liquid cooling medium is allowed to flow through the microchannels and directly wet the surface of the laser gain medium to obtain an enhanced cooling effect. The laser gain medium remains attached and conforming to the substrate since the cooling medium is maintained at a pressure much lower than the pressure of the atmosphere in which the laser module is immersed.
In another preferred embodiment, the rigid substrate is fabricated of material optically transparent to the pump radiation and the pump radiation is delivered to the laser gain medium through the substrate. In this case, a cooling medium transparent to the pump radiation is used.
In yet another alternative preferred embodiment, the substrate is formed by a multi-piece assembly comprising a rigid backing plate having a plurality of headers for communicating a cooling medium therethrough. A manifold plate has a first surface thereof disposed against one surface of the backing plate, and a heat exchanger plate is disposed against a second surface of the manifold plate. The manifold plate and the heat exchanger plate each include flow passages for allowing the cooling medium to be communicated ultimately to the heat exchanger plate where it is used to cool a laser disk medium disposed against a surface of the heat exchanger plate.
Additional preferred embodiments of the present invention incorporate laser gain media which is preformed with a predetermined curvature while in an xe2x80x9cunconstrainedxe2x80x9d condition before being secured to a substrate. During operation, as one surface of the laser gain medium heats up, the opposite surface cools down, and the tensile and compressive stresses experienced on the opposite sides of the medium are relieved as the medium reaches an operating temperature.
The use of optical fibers and hollow ducts for transmitting the optical radiation from one or more optical pump sources into the laser gain medium is also disclosed.
The various preferred embodiments of the laser module of the present invention enable a solid state, high-powered laser to be constructed which has excellent beam quality. Power scaling to tens and possibly hundreds of killowatts is realizable because of the significantly increased cooling of the laser module. This cooling allows the laser module to operate at significantly increased power without overheating. The attachment of the laser medium to the substrate using the pressure differential between the coolant and the surface of the disk further eliminates the attachment-induced thermal stresses of prior art designs and allows the laser module of the present invention to operate at significantly increased power without fracturing the laser gain medium. The invention can also be used as a building block for construction of a laser oscillator or laser amplifier.